step stencil design when 01005 - electronics manufacturing and … · 2011-07-12 · step stencil...

Step Stencil design when 01005 and 0.3mm pitch uBGA’s coexist with RF Shields

William E. Coleman Ph.D.

Photo Stencil

Colorado Springs, CO Abstract

Some of the new handheld communication devices offer real challenges to the paste printing process. Normally, there are

very small devices like 01005 chip components as well as 0.3 mm pitch uBGA along with other devices that require higher

deposits of solder paste. Surface mount connectors or RF shields with coplanarity issues fall into this category. Aperture

sizes for the small devices require a stencil thickness in the 50 to 75 um (2-3 mils) range for effective paste transfer whereas

the RF shield and SMT connector would like at least 150 um (6 mils) paste height. Spacing is too small to use normal step

stencils.

This paper will explore a different type of step stencil for this application; a “Two-Print Stencil Process” step stencil. Here is

a brief description of a "Two-Print Stencil Process". A 50 to 75 um (2-3 mils) stencil is used to print solder paste for the

01005, 0.3 mm pitch uBGA and other fine pitch components. While this paste is still wet a second in-line stencil printer is

used to print all other components using a second thicker stencil. This second stencil has relief pockets on the contact side of

the stencil any paste was printed with the first stencil.

Design guidelines for minimum keep-out distances between the relief step, the fine pitch apertures, and the RF Shields

apertures as well relief pocket height clearance of the paste printed by the first print stencil will be provided.

Introduction

A printing challenge exists when very small devices coexist on a PCB with components requiring either high volume or high

paste height. When printing solder paste for 01005 and / or 0.3 mm pitch uBGA a thin stencil, normally (50 to 75 um) thick

is preferable to achieve sufficient paste transfer efficiency and low paste volume dispersion. If a 100 um thick stencil is used

to print these small devices, the area ratio is less than 0.5 which is lower than the recommended value for both Laser-Cut

stencils and E-FAB Electroform stencils. It is very common to print solder paste for RF shields for hand-held devices.

Normally, higher solder paste deposits are required because of coplanarity issues with RF shields. To achieve these higher

solder paste deposits a thicker stencil is needed. A step stencil(1)

is normally used to achieve different solder paste heights.

However in many hand-held devices the spacing between the small pitch components and the RF shield is normally very

small, in many cases as small as 500 um (20 mils). The design guideline1 for a normal step-down stencil is that the aperture

in the step-down region be positioned at least 890 um (35 mils) from the step-up wall for every 25um (1mil) of step-up

thickness. Otherwise the squeegee blade (metal or rubber) does not do a very job of depositing paste into the lower level

apertures. In the case of RF shields a stencil thickness of 150 to 175 um (6 to 7 mils) may be required to achieve the desired

solder paste height. The height difference; 75um (3 mils) thick stencil for the small components and 150um (6 mils) thick

stencil, prevents a normal step stencil to be used for this application.

However, a “Two-Print Stencil Process” allows for very small spacing between the apertures of the smaller components and

apertures of the RF shield. The objective of this study is to determine the smallest spacing between small component

apertures and RF shield apertures for the Two-Print stencil process. Another objective is to determine the minimum

clearance between the relief pocket of the 2nd

print stencil and the 1st print stencil without paste smearing in the relief pocket.

Two-Print Stencil Test Plan

The Two-Print stencil process consists of two stencils. The 1st print stencil is used to print 0.3 mm pitch uBGA (198 um

circular apertures), 0.4 mm pitch uBGA (244 um circular apertures), 01005 chip components (178 um circular apertures), and

0201 chip component apertures (300 um square apertures). Two 1st print stencils were fabricated. Both were E-FAB

Electroform stencils; one was 50 um (2 mils) thick and the other was 75 um (3 mils) thick. The 2nd

print stencil is used to

print RF shields and a SMT connector and some large capacitor apertures. Four 2nd

print stencils were manufactured with

four different stencil thicknesses; each stencil had a relief pocket etched in the area where solder paste was printed with the

1st stencil. The design is summarized in Table 1.

As originally published in the IPC Printed Circuit Expo, APEX & Designer Summit Proceedings.

Table 1 Two-Print Stencil Test Plan:

1st print stencil is an E-FAB Electroform stencil and prints solder paste for 0.3 mm and 0.4 mm uBGA’s, 01005 and 0201

chip components.

Two stencils were manufactured:

50 um (2 mil) thick E-FAB stencil

75 um (3 mil) thick E-FAB stencil.

2nd

print stencil is a Laser-Cut / Chem-Etch stencil with apertures for RF Shields, capacitors, and a SMT connector. This 2nd

print stencil has relief pockets anywhere paste is printed by the 1st print stencil.

Four 2nd

print stencils were manufactured:

8 mil thick with a 6 mil deep relief pocket




Design of the aperture spacing between 1st and 2

nd print stencil and relief pocket (4 images in the stencil):

Image 1 250 um (10 mil) spacing between apertures in 1st and 2

nd stencil


nd stencil


nd stencil


nd stencil

The manufacturing process for the 2nd

print stencils was to Chem-Etch the Relief pockets in the metal foil then to Laser-Cut

the apertures for the RF shields and other devices in this stencil. The 2nd

print stencil is designed to have different spacing’s

between the apertures in the 1st print stencil and the apertures in the 2

nd print stencil. This design is shown in Figure 1.

2nd

Print Stencil showing apertures and Relief Pockets

Figure 1. Schematic Layout of 2

nd Print Stencil

As seen the spacing is 250 um (10 mils) for Image 1, 500 um (20 mils) for Image 2, 750 um (30 mils) for Image 3 and 100

um (40 mils) for Image 4. Figure 1 is a schematic drawing of the aperture layout and relief pockets for the 2nd

print stencil.


As seen in Figure 1 there is 0 swell for Image 1. This means that the relief etch pocket edge was designed to line up with the

edge of the apertures in the 1st stencil. No etch compensation was used for this relief pocket. When the pocket is etched it

not only etches down but it also etches out making the pocket larger. The increase in pocket size depends on how deep the

pocket is etched. The increase in pocket size is about ½ of the depth of the relief pocket. As an example in the case of the

175 um (7 mil) thick stencil with a relief pocket 125 um (5 mils) deep the increase in size of the relief pocket is 64 um (2.5

mils). The layout of the 1st print stencil is shown Figure 2.

Figure. 1

st Print Stencil showing aperture sizes and aperture position

A photograph of the 1st print E-FAB stencil is shown in Figure 3.

Figure 3. 1st Print E-FAB Stencil


Figure 4 shows a picture of the 2nd

print stencil, a Laser-Cut / Chem-Etch stencil, showing the apertures along with the relief

pockets.

Figure 4. 2nd

Print Laser-Cut Chem-Etch Stencil

Print Results

The test boards used in this experiment were bare copper clad FR4 with 3 fiducials for registration between the 1st print and

2nd

print stencil.

Table 2 Printer Set-Up

Printer: Speedline Momentum

Squeegee: E-Blade Electroformed blade

Board: Blank Copper Clad FR4 with Fids

Print Speed: 1 inch / sec

Print Pressure: 11 lbs.

Paste: Type 5 Heraeus BD72 SAC 305

Print Sequence: Print 1st print stencil then immediately print

2nd

print stencil while 1st print still wet

A Speedline Momentum printer, along with E-Blade Electroformed squeegee blades was used. The printer set-up and

Printing test procedure are shown in Table 2.

Figure 5 shows solder bricks printed with the 1st and 2

nd stencil for Image 1 with 250 um (10 mils) spacing between the 1

st

and 2nd

print. This print sequence used a 50 um (2mil) thick E-FAB 1st print stencil and a 175 um (7 mil) Laser-Cut / Chem-

Etch with a 125 um (5 mil) relief pocket 2nd

print stencil.

Note that the spacing between the .3 mm uBGA and the RF shield is 250 um (10 mils). Figure 6 shows the same print

sequence but for the 01005 chip component solder bricks next to the FR shield solder brick. Spacing is 250 um (10 mils) but

a slight smearing of the 01005 solder brick can be seen.


Figure 5. Solder Bricks .3 mm uBGA Image 1 Figure 6. Solder Bricks 01005 Image 1

Figure 7 shows a close up of the 2nd

print stencil which is 175 um (7 mils) thick with a 125 um (5 mil) deep relief pocket.

Notice that the spacing from the wall of the relief pocket is 175 um (7 mils) to the RF shield aperture. This is as expected for

the Image 1 since the relief pocket increased about 64 um (2.5 mils) from the original starting position of 250 um (10 mils)

from the RF shield aperture.

Figure 7. Relief Pocket for .3 mm uBGA Image 1

Figure 8 shows the same print sequence for Image 2, where the 0.3 mm uBGA apertures were spaced 500 um (20 mils) from

the RF shield aperture. There is no hint of smearing for this configuration.

.3mm uBGA paste bricksRF Shield paste brick

10 mil spacing

Solder Bricks 1st Stencil 2 mil, 2nd Stencil 7 mils

with 5 mil relief pocket for Image 1Solder Bricks 1st Stencil 2 mil, 2nd Stencil 7 mils

with 5 mil relief pocket for Image 1

RF Shield

paste brick

01005

paste brick

10 mils

Slight Smearing

By relief pocket

2nd Stencil- 7 mil thick 5 mil relief pocket

Relief pocket for .3mm pitch uBGAImage 1

RF Shield Aperture Relief Pocket 5 mil deep

7 mils


Figure 8. Solder Bricks .3 mm uBGA Image 2

Solder Paste Volume Results

The solder paste volume measurements were performed at Speedline using a Koh Young paste volume-measuring machine.

The author wished to extend a special thanks to Vatsah Shah for making the measurements.

Table 3

Paste Volume Measurement Summary

1st Print Stencil

Component Number of Solder Bricks Measured

01005 406

0.3 mm pitch uBGA 2048

0.4 mm pitch uBGA 2048

0201 480

2nd

Print Stencil

Component Number of Solder Bricks Measured

5 pin Connector 16

RF Shield 1 24

RF Shield 2 28

Table 3 summarizes the solder bricks measured for each component type in the 1st print stencil and the 2

nd print stencil.

Figures 8 through Figure 11 show a comparison of the 50 um (2mil) thick E-FAB stencil and the 75 um (3 mil) thick E-FAB

stencil for total solder paste volume, % solder paste volume compared to theoretical volume of the stencil aperture, %

standard deviation of the solder paste volume from the mean value, and solder paste height. It is noteworthy, as seen in

Figure 9, that for the smallest component, 01005 chip components with 178 um (7 mil) apertures, the volume of solder paste

for the 50 um (2 mil) thick stencil is almost as much as for the 75 um (3 mil) thick stencil.


Figure 9. Paste Volume Comparison 2 vs. 3 Figure 10. % Paste Volume Comparisons 2 vs. 3

Figure 10 shows that the % paste release is greater for all four components for the 50 um (2 mil) thick stencil compared to the

75 um (3 mil) thick stencil. As seen in Figure 11, the % standard deviation is also smaller for the 50 um (2 mil) stencil than

for the 75 um (3 mil) stencil; especially for the 01005 chip component solder paste. Figure 12 shows the solder paste height

comparison.

Figure 11. % Standard Deviation Comparison 2 vs. 3 Figure 12. Paste Height Comparison 2 vs. 3

Similar solder paste volume measurements were recorded for the 175 um (7 mil) thick stencil and the 200 um (8 mil) thick

stencil. These results are shown in Figures 13 through Figure 16.

Paste Volume Comparison 2 mil thick

Stencil vs 3 mil thick Stencil

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

7 mil C 7.8 mil C 9.6 mil C 12 mil Sq

01005 .3mm uBGA .4mm uBGA 0201

Device Type

Paste

Vo

l cubic

(um

) x

1M

Vol 2 mil

Vol 3 mil

% Paste Volume Comparison 2 mil thick

Stencil vs. 3 mil thick Stencil

0

10

20

30

40

50

60

70

80


01005 .3mm

uBGA

.4mm

uBGA

0201

Device Type

% P

as

te V

olu

me

% Vol 2 mil

% Vol 3 mil

% Std Dev Comparison 2 mil thick Stencil

vs. 3 mil thick Stencil

0

10

20

30

40

50

60


01005 .3mm

uBGA

.4mm

uBGA

0201

Device Type

% S

td D

ev % Std Dev 2 mil

% Std Dev 3 mil

Height Comparison 2 mil thick Stencil vs.

3 mil thick Stencil

0

10

20

30

40

50

60


01005 .3mm uBGA .4mm uBGA 0201

Device Type

He

igh

t in

um

Height 2 mil

Height 3 mil


Figure 13. Paste Volume Comparison 7 vs. 8 Figure 14. % Paste Volume Comparisons 7 vs. 8

As seen from Table 3, the number of data points was much smaller for these two stencils compared to the thinner stencils.

As seen in Figure 14, the % standard deviation is much smaller for these larger apertures compared to the smaller apertures in

the thinner stencils.

Additional Testing

The prior “Two-Print Stencil Process” dealt with two sets of stencils; (1) a 50 um (2mil) thick 1st print stencil used with a 175

um (7 mil) thick 2nd

print stencil having a 125 um (5 mil) deep relief pocket and (2) a 75 um (3 mil) thick 1st print stencil used

with a 200 um (8 mil) thick 2nd

print stencil having a 150 um (6 mil) deep relief pocket. In order to establish the minimum

clearance height required between the 1st print stencil and the thickness of the relief pocket of the 2

nd print stencil a third set

of stencils was tested. A 50 um (2 mil) thick 1st print stencil was used with a 125 um (5 mil) 2

nd thick 2

nd print stencil having

a 75 um (3 mil) deep relief pocket. Print results for this test are shown in Figures 17 to Figures 19 for the Image 1

configuration [250 um (10 mils) spacing between apertures in the 1st and 2

nd print stencils]. Figure 17 shows the position of

the 0.3 mm uBGA solder paste bricks with respect to the RF shield solder paste bricks.

Figure 15. % Standard Deviation Comparison 7 vs. 8 Figure 16. Paste Height Comparison 7 vs. 8

% Paste Volume Comparison 7mil thick

Stencil vs. 8 mil thick Stencil

65

70

75

80

85

90

13 x 78 mils 28 x 71 mils 35 x 114 mils

5 Pin Conn RF aper 1 RF aper 2

Device Type

% P

as

te V

olu

me

% Vol 7 mil

% Vol 8 mil

Paste Volume Comparison 7 mil thick Stencil

vs 8 mil thick Stencil

50

100

150

200

250

300

350

400

450

500



Device Type

Paste

Vol c

ubic

(um

) x 1

M

Vol 7 mil

Vol 8 mil

% Std Dev Comparison 7 mil thick Stencil

vs. 8 mil thick Stencil

0

1

2

3

4

5

6

7



Device Type

% S

td D

ev % Std Dev 7 mil

% Std Dev 8 mil

Height Comparison 7 mil thick Stencil vs.

8 mil thick Stencil

0

20

40

60

80

100

120

140

160

180



Device Type

He

igh

t in

um

Height 7 mil

Height 8 mil


Figure 17. .3 mm uBGA solder bricks 2 mil with 5 mil Figure 18. Magnified view of 17 showing smearing

Figure 18 is a close-up of the same image. This image shows slight smearing in the corner of the uBGA array caused by

contact with the corner of the relief pocket. Figure 19 shows solder bricks for the 01005 chip component with respect to the

RF shield solder brick. Again slight smearing is observed on the solder brick closest to the relief pocket. Figure 20

demonstrates that no smearing was observed when the spacing between apertures in the 1st stencil and 2

nd stencil was

increased to 500 um (20 mils). It can be concluded that with a spacing as low as 25 um (1 mil) between the 1st print stencil

thickness and the depth of the relief pocket on the 2nd

stencil no paste smearing occurred during the 2nd

print as long as the

clearance between apertures is between 250 um (10 mils) and 500 um (20 mils). Although not specifically confirmed by

tests, I believe a safe keep-out spacing between apertures in the 1st print stencil and the 2

nd print stencil is 380 um (15 mils).

This is based on the fact that only the outer halves of the 0.3 mm uBGA solder brick touched the relief pocket at a spacing of

250 um (10 mils) as seen in Figure 18.

Figure 19. 01005 solder bricks, slight smearing Figure 20. 0.3mm uBGA solder bricks with 20 mil spacing

Conclusion

It has been demonstrated that “A Two-Print Stencil Process” is an effective solution to print solder paste when different paste

heights are required. The spacing between apertures requiring different heights can be as small as 380 um (15 mils). It has

also been demonstrated that the depth of the relief pocket in the 2nd

print stencil can be as low as 25 um (1 mil) more than the

thickness of the 1st print stencil. It should also be noted that aperture spacing between apertures in the 1

st and 2

nd print stencil

is independent of the thickness of the 2nd

print stencil. This is very useful when very thick paste heights are required from the

2nd

print stencil. The “Two-Print Stencil Process” offers a new solution to cell phone and hand-held SMT assemblers. Thick

solder paste for RF shields can be printed very close to solder paste bricks for very small devices like 0.3 mm uBGA and

01005 chip components. A design decision must be made as to which apertures are to be included in the 1st print stencil and

which apertures are to be included in the 2nd

print stencil. As a secondary outcome of this testing it is seen, for the particular

print set-up, the standard deviation of the 50 um (2 mil) thick 1st print stencil is significantly lower that the thicker 75 um (3

mil) 1st print stencil. It is also observed that generally, the standard deviation for the 1

st print stencils is lower for the larger

apertures and the thinner stencil.



RF Shield paste brick .3mm uBGA paste bricks

10 mil spacing



01005

paste brick

RF Shield

paste brick

10 mils

Slight Smearing due to

contact with relief pocket



No sign of smeared solder paste bricks

.3mm uBGA paste bricksRF Shield paste brick

20 mil spacing


Design Guidelines

General design guidelines are shown in Table 3.

Future Work

The work presented here had relief pockets positioned in close proximity to apertures in the 1st and 2

nd print stencils. The

relief pockets encompassed the entire area of the device printed in the 1st print stencil. The largest relief pocket was 6.8 mm

x 6.8 mm (0.270” x 0.270”). There was no support issue regarding the 2nd

print stencil flexing down to touching solder

bricks printed by the 1st stencil, even when there was only 25 um (1 mil) clearance.

As a future project it should be determined how large the relief pocket could be designed without flexing to touch 1st print

solder bricks. The future project should also address the size and spacing of support pillars between apertures when the 1st

print stencil has a high density of apertures.

Acknowledgements

The author would like to extend his appreciation and thanks of Vatsal Shah of Speedline for his contribution in measuring the

solder paste volume.

References (1) William Coleman and Michael Burgess “Step Stencils”, Global SMT and Packaging, October 2006


step stencil design when 01005 - electronics manufacturing and … · 2011-07-12 · step stencil...

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